This invention relates to a laminated carbon cathode for use in the production of aluminum by electrolytic smelting.
A cell, or pot, for the production of aluminum by electrolytic smelting usually includes of a rectangular, low steel shell. The bottom and sides of this shell are, on the inside, lined with heat-insulating refractory bricks. On the high temperature side, that is on the inside of the heat insulation, the shell has a carbon lining. This lining is in the form of a shallow vessel which holds the bath and the aluminum that is precipitated during smelting. Inside the carbon lining there are steel bars, so-called cathode bars, to provide the electrical connection between the carbon cathode and external busbars.
The bath used for the electrolytic smelting of aluminum has a temperature of around 1000.degree. C. and is aggressive. This makes the greatest demands on the lining of the smelting vessel, while at the same time, the bottom must be a good conductor of electricity. A large number of compounds e.g. diodes, nitrides and carbides, have been tested as lining materials, but the choice is still dominated by various types of carbon.
The selection of carbon materials for cathodes must take into account price and resistance against impregnation/penetration by compounds in the bath. Decisive for selection is the life of the cathode and the voltage drop through it.
It has now been found that a more or less graphitised cathode exhibits a higher resistance against impregnation and penetration by bath and metal, while at the same time its electrical conductivity is better than that of traditional carbon products on an anthracite base.
In many respects, electrodes of pure graphite would be preferable, but production capacity and price preclude a general adoption of pure graphite cathodes.
Carbon linings are built up of carbon blocks placed alongside one another. They are bonded together by various types of adhesive or tamping paste which is pressed into the seams (slots) between the blocks.
It is these seams which are the weakest element in the carbon lining. The final curing, or hardening, of these seams takes place during the starting of the cell, and it is difficult to achieve optimum heat treatment. The tamping paste also contains volatile substances, with the result that the paste in the slots, after the thermal treatment during the start of the cell, tends to shrink and become porous, and more permeable than the rest of the carbon lining.
Bath and molten metal can penetrate through faulty slots between the carbon blocks, impairing the insulating properties of the refractory lining and attacking the cathode bars. When a pot produces aluminum with unwanted iron and silicon content, this is a warning that the cell is reaching the end of its operating life.
A further process which can help to reduce the operating life of a cell is the oxidation of the cell's carbon side-lining caused by air entering through the holes in the side of the steel shell for the cathode bars.